1. Field of the Invention
The present invention is directed to a radio-frequency antenna for a magnetic resonance system, of the type having a basic element formed by two end rings and a number of antenna rods, the end rings being arranged concentric to an antenna axis and being axially offset relative to one another, and wherein the antenna rods are arranged around the antenna axis and connect the end rings to one another.
2. Description of the Prior Art
Radio-frequency antennas of this type are generally known as birdcage resonators. In such a birdcage resonator, capacitors are arranged in the end rings and/or in the antenna rods. The radio-frequency antenna is tuned such that it forms a resonant circuit at a predefined or pre-definable operating frequency of the radio-frequency antenna.
Such radio-frequency antennas are also known from the English Abstract of Japanese Application 01 075 951 and European Application 1 085 337. In the Japanese patent, the antenna rods are short-circuited at an end ring by further antenna rods that proceed in the plane of the end ring in order to homogenize the sensitivity profile of the antenna. In the European application, the radio-frequency antenna has a displaceable conductor loop provided with a tuning capacitor, this conductor loop surrounding the antenna rods and being inductively coupled to the end rings in order to thus tune the frequency of the antenna.
Magnetic fields emitted by the radio-frequency antenna excite nuclear spins of an examination subject (often a human) into resonance. The resonant signals are then acquired and evaluated. The acquisition of the resonant signals can ensue with the same antenna.
The radio-frequency antenna can be alternatively fashioned as a whole-body coil or as a local coil for the examination of the head or extremities of a human. Local coils are employed in order to achieve a significantly better signal-to-noise ratio than with an overall or whole-body antenna.
In order to optimize the signal-to-noise ratio, radio-frequency antennas are optimized in terms of geometry and transmission and/or reception profile for the different body regions. An optimally good matching to the anatomy of the patient should ensue. Further, an optimally high filling factor of the coil is desirable.
Compromises are always necessary with respect to the optimization of the transmission or reception profile in the case of conventional local coils. A radio-frequency antenna that tapers toward the end (what is referred to as barrel shape) would be desirable for an optimum imaging, however, such a radio-frequency antenna exhibits the disadvantage that it is invariable and closed at one side.
An object of the present invention is to provide a radio-frequency antenna wherein a high filling factor can be achieved in a simple way.
This object is achieved in accordance with the invention in a radio-frequency antenna having a radio-frequency mirror that is galvanically separated from the end rings and the antenna rods but is inductively coupled to them, so that a magnetic resonance excitation signal emitted by the radio-frequency antenna and/or a magnetic resonance signal received by the radio-frequency antenna is axially limited at one side.
As a result, a variation of the positioning of the radio-frequency mirror is possible.
Examination steps alternate with treatment steps in various applications, for example stereotaxy. In this case, a closed coil shape is not possible since no access to the patient would be possible when the coil is applied.
In an embodiment wherein the radio-frequency mirror is mechanically releasably connectable to the basic element, a nearly optimum imaging or free access to the patient can be achieved as selectable alternatives with the radio-frequency antenna.
Given a radio-frequency mirror connected to the basic element, the radio-frequency antenna has a transmission profile and reception profile that nearly achieve the profile of the optimum barrel shape. With the radio-frequency mirror removed, in contrast, access to the patient is preserved.
A even greater optimization due to a higher degree of coil utilization is possible when the radio-frequency mirror is connectable to the basic element in a number of axial positions.
An infinitely variable adjustment is even possible in an embodiment wherein the radio-frequency mirror is connectable to the basic element in an axially displaceable manner.
In an embodiment wherein the antenna rods proceed parallel to the antenna axis, the spacing of the radio-frequency mirror from the antenna rods is independent of its axial positioning.
The number of antenna rods in the inventive radio-frequency antenna typically amounts to between four and sixteen, and is usually even.
In the simplest case, the radio-frequency mirror can be fashioned as a metallic plate or sheet proceeding transversely relative to the antenna axis. Fewer low-frequency eddy currents arise, however, when the radio-frequency mirror is fashioned as a system of interconnects that are galvanically separated from one another and proceed concentric to the antenna axis, the system being disposed transversely relative to the antenna axis.
In this latter instance, the interconnects can be galvanically interrupted (have gaps), the interruptions being bridged by capacitors and/or the interconnects can be bridged relative to one another with capacitors. Either measure reduces the low-frequency eddy currents even though the interconnects are short-circuited in terms of radio frequency.
In the optimum case, the radio-frequency mirror is fashioned as an interconnect that proceed helically around the antenna axis. In this design as well, the interconnect can be radially bridged with capacitors.
For mechanical stability, the radio-frequency mirror preferably is arranged on an electrically non-conductive carrier. The carrier can be fashioned, for example, as a circuit board.
The inventive radio-frequency antenna preferably is fashioned as a head coil. In this case, it has an essentially cylindrical examination volume with a diameter of 18 to 30 cm and a length of 18 to 30 cm. The inventive antenna also can be fashioned as a whole-body coil with a diameter of 50 to 80 cm and a length of 30 to 150 cm.